Paul, speaking at the Southern Kentucky Lincoln Day Dinner late Saturday, said that he couldn't "vote to give extra-constitutional powers to the president."

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“I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress. We may want more money for border security, but Congress didn’t authorize it. If we take away those checks and balances, it’s a dangerous thing," he said, according to the Bowling Green Daily News.

Paul's stance makes him the crucial fourth Republican senator to support the resolution, marking a setback for the administration that would force Trump to use the first veto of his presidency.

He elaborated in a Fox News op-ed, published on Sunday evening, that he would vote for the resolution of disapproval when it gets a vote in the Senate.

"Every single Republican I know decried President Obama’s use of executive power to legislate. We were right then. But the only way to be an honest officeholder is to stand up for the same principles no matter who is in power," he added.

Paul added that Trump's decision to declare a national emergency to get funding beyond the $1.375 billion approved in a recent government funding bill "is clearly in opposition to the will of Congress." He added that if Trump was unsatisfied with the amount provided by lawmakers "his only constitutional recourse" would have been to veto the bill.

"I think the president’s own picks to the Supreme Court may rebuke him on this. Regardless, I must vote how my principles dictate. ... I think he’s wrong, not on policy, but in seeking to expand the powers of the presidency beyond their constitutional limits," Paul wrote.

The Senate is expected to vote on a resolution blocking Trump's emergency declaration before leaving town for a weeklong recess set to begin on March 15.

Supporters of the resolution left Washington last Thursday on the brink of having enough votes to block the declaration and force a Trump veto. With all 47 senators who caucus with Democrats expected to support the resolution, they need to flip four Republicans to clinch the 51 votes needed in the Senate.

Roughly a dozen other Republican senators remain on the fence about Trump's emergency declaration, meaning supporters could pick up several additional GOP "yes" votes before the resolution comes to the floor.

“I will announce how I’m going to vote when I know what we’re going to be voting on,” Alexander told reporters.

“I learned a long time ago in the United States Senate it’s not wise to announce how you’re going to vote on a vote you may never have to take,” he added.

Trump has said he will "100 percent" veto the resolution of disapproval if it reaches his desk. He also appeared to warn Republican senators who are thinking about opposing him, arguing that supporting the resolution would be a "very dangerous thing."

“I really think that Republicans that vote against border security and the wall, I think you know, I’ve been OK at predicting things, I think they put themselves at great jeopardy,” he told Fox News’s Sean Hannity last week.

Trump announced that he would declare a national emergency the day after Congress passed a government funding bill that included $1.375 billion for physical barriers along the U.S.-Mexico border, well below the $5.7 billion that he requested. He’s hoping to pull together roughly $8 billion for the border wall by combining the emergency declaration, executive actions and the money from Congress.